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I am quite new to GIS software. I use QGIS. I have a question that might be trivial. I would like to derive the coordinates (Y and X) for an area. Just to make an example: I have two regions A and B. From the shapefile I know which are their coordinates. However, I would need the coordinates for the unique region A+B. Could you please give me some hints on how to do that?

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    What do you mean by "coordinates for an area"? An area does not have coordinates, a point does. – ArMoraer Apr 20 '16 at 16:24
  • Yes you are right. I mean lon and lat for the points determining the region (A+B). It seems to me that field calculator should be used, but I am a bit lost with it. – Dario Apr 20 '16 at 16:43
  • What do you mean by region A+B? Depending on what you mean that, you could do something like a merge or intersection, then extract the coordinates of the vertices. Just make sure that the geoprocessing step that you use is consistent with what you mean by A+B. – Ezra Boyd Apr 20 '16 at 22:40
  • Thanks for your reply. What I mean is the following. Example: In the original shape file I have information about the coordinates to plot Spain an Portugal. I need to plot them as a single area. Therefore, I need the coordinates to plot spain+Portugal as they were only one country. So if I understood correctly, you suggest to merge them and to extract the coordinates of the vertices. This should work for my case. I presume that I can merge them highlighting those two countries on the map. But then how can I extract the coordinates of the vertices? – Dario Apr 21 '16 at 1:14
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From the precisions you provided in the comments, I understand that what you want is actually a polygon merging followed by some kind of vertices coordinates export. This is actually quite simple:

  1. Merging: Select your layer then click on the pen icon to toggle edition mode. Select your two polygons, then go to Edit > Merge Selected Features. This will replace your two polygons with their geometric union (so if you want to keep your old polygons, make a copy of the layer first). This will work well if there are no gaps or overlap between your polygons.

  2. Coordinates export: there are (at least) 2 options. You can right click on the layer, then "Save As..."; in the "Format" field, choose CSV. In "Layer options", "Geometry" field, choose "AS_WKT". Alternatively, you can install and use the MMQGIS plugin (see this answer for reference).

  • Union still has the border in the middle. Dissolve using Dissolve all creates one polygon covering all elements. – AndreJ Apr 21 '16 at 5:38
  • @AndreJ: you're absolutely right: I confused union with the merge tool. I'll correct the answer. – ArMoraer Apr 21 '16 at 5:46

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